Place 3 cups watercress in blender. Pour half of warm broth mixture over watercress and blend until smooth. Add half and half and blend until combined.

Transfer soup to bowl. Puree remaining broth mixture in blender until smooth; stir into soup in bowl. If more watercress flavor is desired, puree 1/2 cup watercress with 2 cups soup in same blender. Repeat with more watercress, if desired. Season soup to taste with salt and pepper. Cover and chill until cold, at least 4 hours. DO AHEAD Can be made 1 day ahead. Keep refrigerated.

For green onion cream:

Whisk crème fraîche, white and pale green parts of green onions, and all remaining ingredients in small bowl. DO AHEAD Can be made 1 day ahead. Cover and chill.